Core network is often required for transmission in conventional wireless communication technology but resulting in the problem of core network congestion. Therefore, a direct communication technology that works without going through a base station has been receiving more attention lately. Device-to-device (D2D) communication is a technology that enables direct communication among user equipments (UEs) in which use of the base station (e.g., eNodeB or enhanced NodeB) for forwarding data is not required. Application of D2D communication may be classified as proximity-based service (PBS; ProSe; or known as Proximity Communication), which is a service applicable only for the user equipments near one another. D2D communication mainly includes three major procedures: Device Discovery, D2D peering and communication.
On the other hand, public land mobile network (PLMN) established by each mobile communication provider could provide a communication service for users. Communication interface and frequency band used by each mobile communication provider are different from the others to avoid issues such as mutual interference and the like. FIG. 1 is a schematic diagram of a communication system 100. A base station 110 and a base station 120 respectively belong to PLMNs of different mobile communication providers. Various user equipments 11 to 13, 21 to 23 and 31 to 33 are included within a communication range R1 of the base station 110 and a communication range R2 of the base station 120. Nonetheless, the user equipments may use services of different mobile communication providers, and the PLMN of each of the mobile communication providers may also include a communication interface and a carrier frequency band which are different from the others. For example, the user equipments 11 and 12 may use a mobile network service provided by the base station 110 by the carrier frequency band 1800 MHz while the user equipment 13 uses a mobile network service provided by the base station 120 by the carrier frequency band 700 MHz, and thus the user equipment 11 cannot learn of information related to the user equipment 13 across the frequency bands. As a result, the D2D communication is still only available for the user equipments belonging to the same mobile communication provider at present since the user equipments are unable to overcome restrictions across the mobile communication providers and the carrier frequency bands for conducting the D2D communication.